The Boomtown Rats singer will join Mr Davis on the campaign trail in his extraordinary 'freedom' by-election.

His endorsement is embarrassing for the Government, since Mr Geldof worked closely with Tony Blair and Gordon Brown on the issues of Third World aid and debt.

Bob Geldof: Backing David Davis

Just days ago, he was pictured in a clinch with Cherie Blair at a dinner in London for Nelson Mandela's 90th birthday.

But he said last night: 'What is the point of Britain without its freedoms? What terrorises the terrorists is our civilisation.

'I am delighted to support this campaign and I will be speaking on it in Hull on Friday.'

Mr Davis said: 'I am delighted that Bob Geldof is supporting our campaign, which stretches across all parties and is vital to the freedom of our country.'

Mr Davis quit as shadow Home Secretary and as an MP in protest at Government plans to introduce six-week detention without charge and other erosions of traditional freedoms.

Mr Cameron has made clear that he disagreed with Mr Davis's decision to stand down because it threatened the stability of his 'top team', though he campaigned for him in his Haltemprice and Howden constituency this week.

Mr Davis will not return to his old job, which has been filled by former shadow Attorney General Dominic Grieve.

Labour has refused to field a candidate in the by-election, which will take place on July 10. Ministers have been doing their best to ignore the contest.

The Liberal Democrats have also stepped aside in the contest because they agree with Mr Davis's stance on liberty.

That has left him fighting a bewildering range of individuals and smaller parties who have thrown their hats into the ring.

They include rape law campaigner Jill Saward, who was a victim of the infamous Ealing Vicarage rape in 1986, who is fighting to campaign for the criminal justice system to place more emphasis on the rights of the victims.

Other candidates include the well-known conspiracy theorist David Icke, Official Monster Raving Loony Party candidate Mad Cow-Girl and Miss Great Britain Gemma Garrett.

Among the other parties which will be on the ballot paper are the Church of Militant Elvis Party and a representative of a campaign group called Make Politicians History.